1. Field of the Invention
The invention relates to a common rail injector for injecting fuel in a common rail injection system of an internal combustion engine, which system has an injector housing with a fuel inlet that is in communication with a central high-pressure fuel reservoir outside the injector housing and with a pressure chamber inside the injector housing, from which fuel subjected to high pressure is injected as a function of the position of a control valve that assures that a nozzle needle movable back and forth and received in a longitudinal bore of the injector axially counter to the prestressing force of a nozzle spring that is received in a nozzle spring chamber, lifts from a seat when the pressure in the pressure chamber is greater than the pressure in a control chamber that communicates with the fuel inlet via an inlet throttle.
2. Description of the Prior Art
In common rail injection systems, a high-pressure pump pumps the fuel into the central high-pressure fuel reservoir, which is called a common rail. From the high-pressure fuel reservoir, high-pressure lines lead to the individual injectors, which are assigned to the engine cylinders. The injectors are triggered individually by the engine electronics. The rail pressure prevails in the pressure chamber and at the control valve. When the control valve opens, fuel subjected to high pressure reaches the combustion chamber, past the nozzle needle that is lifted counter to the prestressing force of the nozzle spring.
In conventional injectors of the kind known for instance from German Patent Disclosures DE 197 24 637 A1 or DE 197 32 802 A1, relatively long nozzle needles are used. In operation, because of the high pressures and the rapid load changes, very strong forces act on the nozzle needle. These forces cause the nozzle needle to be stretched and compressed in the longitudinal direction. This in turn means that the nozzle needle stroke varies as a function of the forces acting on the nozzle needle.
The object of the invention is to furnish a common rail injection system with a small structural volume that is simple in design and can be produced economically. In particular, even at a high nozzle needle speed, good closing performance should be assured.
In a common rail injector for injecting fuel in a common rail injection system of an internal combustion engine, which system has an injector housing with a fuel inlet that is in communication with a central high-pressure fuel reservoir outside the injector housing and with a pressure chamber inside the injector housing, from which fuel subjected to high pressure is injected as a function of the position of a control valve that assures that a nozzle needle movable back and forth and received in a longitudinal bore of the injector axially counter to the prestressing force of a nozzle spring that is received in a nozzle spring chamber, lifts from a seat when the pressure in the pressure chamber is greater than the pressure in a control chamber that communicates with the fuel inlet via an inlet throttle, this object is attained in that the control chamber is defined by a bush that is displaceable, causing a sealing action, on the end of the nozzle needle remote from the combustion chamber and that is kept in contact against the injector housing with the aid of the nozzle spring. The bush offers the advantage that the control chamber and the nozzle spring chamber can be combined on the end remote from the combustion chamber of the nozzle needle, without the volume of the control chamber depending on the structural space of the nozzle spring. It is therefore possible to build in a nozzle spring with high spring rigidity, which assures good closure of the nozzle needle. As a result, the injection time and the instant of injection can be defined exactly. Furthermore, the control chamber can be made quite small, which leads to a rapid response behavior of the injector of the invention. There is also a relationship between the maximum attainable nozzle needle speed and the nozzle needle diameter. To achieve elevated nozzle needle speeds, which is important especially upon needle closure, the nozzle needle diameter must be reduced. For a closing speed of 1 m/s, a needle diameter below 3.5 mm is required, for an acceptable control quantity. This is technologically quite complicated and therefore expensive. According to the present invention, the nozzle needle diameter can be selected freely and is not dependent on the dimensions of the nozzle spring. In comparison to convention nozzle needles, the length can be reduced considerably, which contributes to an exact stroke stop.
One particular type of embodiment of the invention is characterized in that a biting edge is embodied on the face of the bush that is in contact with the injector housing. As a result, it is attained that the control chamber embodied in the interior of the bush remains separated from the nozzle spring chamber that surrounds the bush.
A further particular type of embodiment of the invention is characterized in that the inside diameter of the bush is less than or equal to a guide diameter at the nozzle needle. The smaller the control chamber volume can be selected to be, the more readily the injector reacts. According to the present invention, the inside diameter of the bush and the corresponding outside diameter at the nozzle needle can be made much smaller than in conventional injectors.
Another particular type of embodiment of the invention is characterized in that the fuel inlet communicates with the pressure chamber via the nozzle spring chamber, and that the nozzle needle is guided between the nozzle spring chamber and the pressure chamber. This offers the advantage that the nozzle needle guide no longer has any sealing function. This makes the demands in terms of quality of the guide less stringent, leading to economies in production. Since the same pressure prevails on both sides of the guide, guide leakage no longer occurs.
A further particular type of embodiment of the invention is characterized in that the nozzle spring chamber communicates with the pressure chamber via a bore. As a result, the entire circumference of the nozzle needle can be utilized for guide purposes.
A further particular type of embodiment of the invention is characterized in that at least one flat face, past which fuel from the nozzle spring chamber can reach the pressure chamber, is embodied on the nozzle needle between the nozzle spring chamber and the pressure chamber. This type of embodiment offers advantages especially with regard to the high-pressure strength.
Further particular types of embodiment of the invention are characterized in that the inlet throttle is integrated with the bush, the nozzle needle or the injector housing. The inlet throttle serves to prevent pressure surges in operation.
A further particular type of embodiment of the invention is characterized in that the bush has a collar on its end remote from the combustion chamber. The collar forms a first abutment for the nozzle spring.
A further particular type of embodiment of the invention is characterized in that a step that forms a stop for a spring plate is formed on the nozzle needle. The spring plate forms a second abutment for the nozzle spring.
A further particular type of embodiment of the invention is characterized in that a circumferential groove is recessed out of the nozzle needle, and in this groove a retaining ring which forms a stop for a spring plate is braced. In this type of embodiment, the outside diameter of the nozzle needle in the control chamber and the guide diameter of the nozzle needle between the nozzle spring chamber and the pressure chamber can be the same size. This is advantageous in production, for instance by means of lapping.
A further particular type of embodiment of the invention is characterized in that the retaining ring is in two parts, and in the assembled state it is fixed by the spring plate. As a result, loosening of the spring plate in operation is prevented in a simple way.
A further particular type of embodiment of the invention is characterized in that the nozzle needle stroke is defined by the spacing between the bush and the spring plate. This purely mechanical nozzle needle stroke end stop offers the advantage that the nozzle needle stroke is exactly replicable. As a result, the course of injection can be shaped reliably. So-called hydraulic sticking is avoided.
A further particular type of embodiment of the invention is characterized in that the nozzle needle stroke and the nozzle spring prestressing can be adjusted with the aid of spacer elements, which are disposed between the spring plate and the stop for the spring plate, and between the nozzle spring and the abutments for the nozzle spring. As a result, the closing performance of the injector can be improved.
A further particular type of embodiment of the invention is characterized in that the nozzle needle stroke is defined by the spacing between the end face, remote from the combustion chamber, of the nozzle needle and the injector housing. This type of embodiment has the advantage of being especially simple to achieve in terms of production technology.
A further particular type of embodiment of the invention is characterized in that recesses, whose dimensions are adapted to the volume of the control chamber, are provided in the end face, remote from the combustion chamber, of the nozzle needle and/or in the opposed face of the injector housing. In order in injector operation to achieve the most linear possible quantity performance graph, it is useful not to embody the nozzle needle stroke purely hydraulically. In a purely hydraulic nozzle needle stroke stop, it can happen that the nozzle needle xe2x80x9cfloatsxe2x80x9d on a pressure cushion in the open position. This can cause vibration of the nozzle needle. The vibration in turn leads to nonlinear quantity performance graphs. Since this is a dynamic motion, the result is a greater tolerance dependency. The vibration of the nozzle needle can depend on the inlet throttle and the outflow throttle, the friction of the nozzle needle guide, the control chamber volume, and so forth. In a purely mechanical stop, although vibration of the nozzle needle is indeed avoided, still a somewhat greater control quantity is required for the purpose. This has an unfavorable effect on the efficiency of the injector. The recesses, which can for instance take the form of cross-shaped slots, creates a xe2x80x9csemi-hydraulicxe2x80x9d stop. The flow cross section that remains upon impact is selected to be precisely large enough that while vibration of the nozzle needle is avoided, still the control quantity at the end stop is lowered as far as possible. In this respect it is advantageous that the injector of the invention has no leakage; that is, without triggering of the injector, no return quantity is generated.
A further particular type of embodiment of the invention is characterized in that in the end face, remote from the combustion chamber, of the nozzle needle at least one axial bore is provided, which communicates with at least one radial bore in the nozzle needle. This type of embodiment has the advantage of being insensitive to mechanical running-in; that is, the flow cross section does not change over the service life.